A horizontal metal extrusion press consists of a press cylinder crosshead, a counter crosshead or platen, tie rods connecting these, a moving crosshead with a press ram, and a receiver for the billet to be extruded, the moving crosshead and the billet receiver being movable along the press axis, and the die which forms the extrusion being inserted in a die holder and being supported on the counter platen via a pressure ring or pressure rings. Great importance is attached to the design of the carrier for the tool set, composed of the die, the die holder and the pressure ring(s), since this has a decisive influence on the service life of the dies, the product quality, the die changing times, the operational reliability and the structure of the metal extrusion press. An important distinctive feature of metal extrusion presses is therefore the design of the tool carriers as slides or rotary heads. So that a change of dies can be prepared or a die can be treated (reconditioning, cleaning, lubricating, cooling, preheating) whilst another die is in use, the slides and the rotary heads are provided with two receivers which can be brought alternately into the working position (company specification P2/317 "Horizontal Rod and Tube Extrusion Presses", Schloemann Aktiengesellschaft, of October 1966; Zeitschrift for Metallkunde 51st year, (1960) no. 1, p. 29-35, "Hydraulische horizontale Strangpressen" von W. Dohrn and H. Schmoll).
Tool slides have the disadvantage that a change or treatment of dies must be carried out alternately on each side of the press. Rotary tool heads have a high spatial requirement resulting from their swivelling circle, and their structural complexity of rotary tool heads is a disadvantage; these disadvantages are compounded by the fact that they require a locking mechanism for the rotary arm when it is in the working position, which locking mechanism supports the rotary arm against axial forces occurring when the receiver is pulled off the shearing tool head and against radial shearing forces. Because of these disadvantages of rotary tool heads, it is preferable to use tool slides, particularly in the modified form of cassette slides in conjunction with a quick changing device for the cassettes, since these allow very short changing times whilst retaining a simple structure and the change is only carried out on one side of the press. In order to receive the tools, the cassettes are provided with a U-shaped opening at the top and are displaceable in a horizontally transversely to the axis of the press between a an extrusion position and a changing position, and to this end the counter platen of the press is provided with a guide frame, comprising an upper and a lower guide rail, on its side facing the press cylinder crosshead. A piston-cylinder unit is connected to the guide frame as a displacement drive for the cassettes, which can be replaced in the changing position by means of a second guide directed transversely to the first guide (EP 0 318 631 A2; firm specification P2/3210 "Standard extrusion press for aluminium", Schloemann-Siemag AG, of April 1981).
In order to secure a die holder axially in a tool carrier (slide, rotary head or cassette), the external diameter of the die holder is increased towards the counter holder by a step, the annular surface of the step being formed by a shoulder, a collar or an annular groove, and the tool carriers are shouldered correspondingly, the corresponding shoulder, the groove corresponding to the collar of the die holder or the bead corresponding to the annular groove of the die holder continuing on the U arms of the tool carrier opening. If, at the end of the extrusion process, the extrusion residue is severed by a shearing blade moved along the front surface of the die and the die holder, the die and the die holder can be lifted in the tool carrier when the shearing blade is moved back, which should not be possible. In order to prevent this, tool slides are closed with locking pieces, which need to be removed and reinserted whenever the die and the die holder are replaced (DE-AS 1 020 951). When using tool slides with two receivers for dies which can be brought alternately into the working position, the change of dies is simplified by a locking bar which is movable to a limited extent in the tool slide between limit stops and closes the tops of the openings for the tool sets in the respective extrusion position (DE-OS 21 34 258).
Before the extrusion residue can be sheared off, the receiver needs to be displaced so that the extrusion residue is exposed. A considerable axial force occurs when the receiver is "stripped off" in this manner, and a tilting moment occurs on the die holder when the extrusion residue is sheared off. Since, on the corresponding surfaces of the die holder and the tool carrier, the die holder is supported only slightly more than in its lower half, eccentric transmission of forces occurring when the receiver is torn off and the tilting moment occurring when the extrusion residue is sheared off lead to high localized surface pressure in the region where the die holder's periphery changes from being supported to being unsupported, with the result that these surfaces are deformed and it is not possible to guarantee the desired seating of the tools in the tool carrier to eliminate tilting. However, a tilting of the tools has the result that shearing off the extrusion residue does not produce a clean cut which is plane-parallel to the front surface of the die and the die holder. Projecting material residues, which can build up during successive extrusion processes, finally prevent the billet receiver from resting against the die holder in a sealing manner. In dies with an antechamber, an unclean cut can lead to air pockets, thus making it difficult to carry out billet-to-billet extrusion and to separate a narrow piece of extrusion containing the weld.
The known locking pieces or locking bars are not suitable for improving the support of the tools in the tool carrier so that tilting can be excluded, since to achieve this the support would need to be designed so as to be free from play and have sufficient dimensions. A locking piece according to DE-AS 1 020 951 can only be inserted and taken out again easily if it is designed with plenty of play. A locking bar according to DE-OS 21 34 258 gives an insufficient supporting surface.